I. Field
The present invention relates generally to data communication, and more specifically to techniques for time-aligning transmissions from multiple base stations in a CDMA communication system.
II. Background
Wireless communication systems are widely deployed to provide various types of communication including voice and packet data services. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access techniques. CDMA systems may provide certain advantages over other types of systems, including increased system capacity. A CDMA system is typically designed to conform to one or more standards, such as IS-95, cdma2000, and W-CDMA standards, which are known in the art.
A CDMA system may be operated to support voice and data communication. During a communication session (e.g., a voice call), a terminal may be in active communication with one or more base stations, which are placed in an “active set” of the terminal. While in soft handover (or soft handoff), the terminal concurrently communicates with multiple base stations, which may provide diversity against deleterious path effects. The terminal may also receive signals from one or more other base stations for other types of transmission such as, for example, pilot references, pages, broadcast messages, and so on.
In accordance with the W-CDMA standard, the base stations are not required to be operated synchronously. When operated asynchronously, from a terminal's perspective, the timing (and thus, the radio frames) of the base stations may not be aligned and the reference time of each base station may be different from that of the other base stations.
While in soft handover, a terminal concurrently receives data transmissions (i.e., radio frames) from multiple base stations. To ensure that the radio frames arrive at the terminal within a particular time window so that they can be properly processed and recovered, the W-CDMA standard provides a mechanism whereby the starting time of the terminal-specific radio frames from each base station to the terminal can be adjusted. Typically, before a new base station is added to the terminal's active set, this base station's timing relative to that of a reference base station is determined by the terminal and reported to the system. The system then instructs the new base station to adjust its transmit timing for the terminal such that the radio frames transmitted from this new base station are approximately aligned in time to the radio frames from the other active base stations.
For the W-CDMA standard, the time difference between a new candidate base station and a reference base station can be reported via an “SFN-SFN observed time difference type 1 measurement” (where SFN denotes system frame number). This measurement includes two parts. The first part provides the chip-level timing between the two base stations, which can be derived by detecting the timing of the pseudo-noise (PN) sequences used to descramble the downlink signals from these base stations. The second part provides the frame-level timing between the two base stations, which can be derived by processing (i.e., demodulating and decoding) a broadcast channel transmitted by the base stations. These two parts are encapsulated into a report message that is transmitted from the terminal to the system.
In certain W-CDMA system configurations, only the chip-level timing is required to properly time-align the radio frames of a newly added base station. This may be true, for example, if the base stations are operated synchronously and the frame-level timing is already known by the system. In this case, requiring the terminal to measure and report the frame-level timing as well as the chip-level timing (as required by the current W-CDMA standard) may degrade performance.